In the sputtering coating process a kind of vapour is created, the atoms or molecules of which hit a substrate to be coated. The vapour is created by bombarding a target with ions derived from a gas, called the sputtering gas, which e.g can be an inert gas such as argon. The ions are created by maidg art electric discharge, thereby producing electrons which ionize the gas. In magnetically enhanced or magnetron sputtering a magnetic field is created in such a way as to trap and concentrate the electrons produced in the electric discharge to form an electron cloud. This electron cloud, which for a suitable design of the magnetic field will be located at the surface of the target and have a high density of electrons, will then cause ionisation of the sputtering gas in the region close to the target surface. The target has a lower electric potential than the region in which the electron cloud is formed and will then attract positive ions to move with a high velocity towards the target. The impact of these ions at the target dislodges atoms from the target material. The dislodged atoms will then move into the region outside the target surface and into all of the space where the discharge is made and the target is located. The atoms will fully be deposited on the ws of said space and thus also on the surface of the substrate.
Due to the relatively high ionisation efficiency of this process using magnetic confinement compared to other sputtering methods, relatively low power levels may be used, whereas at the same time high sputtering rates are achieved. Because electron losses perpendicular to the magnetic field intensity lines are restricted by the suitably designed geometry of the magnetic field, bombardment of the substrate is minimised and the heating of the substrate, in particular of the growing film on the substrate, is significantly smaller than in other sputtering methods. Electron losses in the directions of the magnetic field intensity lines are determined by the combined geometry of the magnetic and electrostatic fields, which can be designed to form a so called mirror confinement of the electrons.
However, magnetron sputtering have some drawbacks compared to other sputtering methods. One of the most important drawbacks is the low utilization of the target and the accompanying effect of obtaining deposited layers having a non-form thickness. This is caused by the localized ionization of the sputtering gas resulting essentially from the low electron temperature. Because of the low temperature and the confinement effects resulting from the geometry of the magnetic and electric fields, electrons which cause the ionization are concentrated in narrow regions above or at some small distance from the surface of the target. These narrow regions are also located between the poles of the magnets used for setting up the magnetic field. In these narrow, localized regions most of the ionization of the sputtering gas occurs. After the ionization the ions move and are accelerated towards the surface of the target, in paths substantially perpendicular to that surface. The location of the ionization regions will thus be mapped on the target surface resulting in a non-uniform erosion or wear of the target which in turn causes that only a restricted portion of the target can be used until it has been eroded through. The amount of ionized gas can be increased by increasing the voltage applied but then the probability of arc formation could be very high.
Magnetically enhanced sputtering is widely used in science and technology such as for coating object with various materials. The most important areas, in which magnetically enhanced sputtering is used, generally comprise magnetron sputtering devices intended for coating of work pieces. Also, in sputtering ion pumps for creating very low pressures a magnetically enhanced sputtering process is used in which the coating of some object is not the primary object, but in the process when fresh sputtered atoms are deposited on wall surfaces of a chamber they will adsorb molecules or atoms of the ionizing gas, lowering the pressure thereof.
Coating by means of sputtering and in particular magnetron sputtering is used within a multitude of technical fields. It can be used to produce anti-corrosion coatings, wear resistive coatings, thermo-resistive coatings, decorative coatings, optical coatings, transparent electroconductive coatings for displays, coatings of polymers with metallic fmims, ferromagnetic coatings for magnetic Memories, superconducting coatings for memory cells, ultrafine cogs for photo and X-ray patterns, hard coatings (carbides, nitrides), resistive coatings, metallisation in electronics and microelectronics, metallisation of RF, HF and UH equipment, etc. Advantageous characteristics of coatings produced by magnetron spring comprise for example a high adhesion to the substrate and a low porosity. Furthermore, magnetically enhanced sputtering will cause only small radiation damages to the substrate to be coated. Owing to the fact that a low temperature of substrate can be maintained during the coating process, also delicate materials can be coated. Magnetically enhanced sputtering allows a high sputtering rate and is also suited for reactive sputtering, in which atoms sputtered from the target combine with atoms in the gas to produce a coating consisting of molecules formed by the combined atoms. Furthermore, it allows sputtering of superconductive materials, sputtering of ferromagnetic materials, sputtering of composite materials and sputtering of materials having high melting temperanes.
Magnetron sputtering is in many respects advantageous compared to other similar coating methods such as electron-beam evaporation and RF-sputtering.
Furthermore, sputtering ion pumps are today used in a lot of different branches of science and technology where a high vacuum is required and used. In science, e.g. in atom physics, nuclear physics such as in particle accelerators, solid state physics, plasma physics for research in thermonuclear fusion, and in different investigations in electronics and microelectronics and in developing processes of deposition of layers for optical devices, for instruments, etc. In technology sputtering ion pumps are used in the processing for manufacturing electronic and amicroelectronic circuits, in industrial particle accelerators producing coatings for optical devices such as lenses and panes, in producing cutting and abrasive tools and in many other fields.
However, conventional sputtering ion pumps as well as other sputtering devices used today have some drawbacks. The most important drawback is the limited discharge power resulting from the fact that a degassing of electrodes can occur owing the heating thereof during the discharge used in the sputtering process. If the discharge power used in the conventional sputtering process is too high, the electrodes will be heated so much, that the rate of degassing of the electrodes exceeds the gas adsorption intensity of the electrodes. This phenomenon is most critical in the pressure range of 10.sup.-2 -10.sup.-5 torr. When starring the operation of a conventional sputtering ion pump in the pressure range of 10.sup.-2 -10.sup.-3 torr, the pump operation is thus characterized by a low voltage between electrodes of about -200 V and a high discharge current. Because of the low voltage the efficiency of sputtering for adsorbing atoms/molecules, also called getter sputtering, is very low and accordingly the pumping speed is also very low. If the discharge current is increased, the temperature of the electrodes will also be increased and consequently also the rate of degassing the electrodes. Also, in the somewhat lower pressure range of 10.sup.-3 -10.sup.-5 torr the discharge current is still too high and it is necessary to arrange a limitation of the discharge power.
As has already been observed, the conventional methods of magnetically enhanced sputtering such as magnetron sputtering and sputtering ion pumps have a number of draw-backs.
Methods have also been proposed in which the power to the sputtering process is provided in discrete or individual pulses. Thus, in U.S. Pat. No. 5,015,493 a process and apparatus are disclosed for coating conductive work pieces, using a pulsed glow discharge. Sputtering and ion plating methods are described. The region of abnormal glow discharge is used in the coating process, which can be magnetically enhanced.